This invention concerns a plough frame comprising at least one body having a plough board and a single symmetrical ploughshare, acting in both ploughing directions, left and right, designed to be hitched to a tractor. The principle is known of variable width for single ploughs and wheel ploughs, achieved by various articulated quadrilaterals which, because of their purpose and asymmetrical designs cannot be applied to the principle of the symmetrical body plough for acting in both ploughing directions. Also, the position of the pivot of the body the farthest forward, taken with respect to the direction of forward movement, is independent of the position of the different articulation axes of the deformable quadrilateral. The principle is known of the symmetrical body plough according to patent FR-A-2635635 by which the different ploughing widths in both ploughing directions are achieved by a bracket arm, one of its ends oscillating on both sides of the vertical longitudinal plane of symmetry of the head, around a first axis of the head located in a vertical plane of the head, and by the other end receiving by a second axis the body support beam which also oscillates with respect to the bracket arm on both sides of the vertical plane passing through the two axes.
Also known from the same patent and from application FR 8905192 is the principle of variable width: the change of the forward body support beam from one ploughing direction to the other and the continuous positions of ploughing width achieved by the means provided by the invention of a ramp of various shapes necessarily participating in its guidance, the angle that the beam makes with the bracket arm varying throughout the movement-of the beam at the different widths for the same depth of ploughing.
Although this type of plough overall represents a large reduction in weight and price compared to the principle of the reversible plough, double acting left and right body, the head of the symmetrical body plough is elaborate and bulky: the ramp or its support having to be large enough which depends on the perpendicularity of the plough when the tractor is in the ridge between furrows and the fact that they must achieve both directions of ploughing, it follows that there is a certain weight and cost of manufacturing. Such a principle for the frame is limited to ploughs with a small number of bodies, the movement of the beam from a point on the guide rod to the central point, only being able to be achieved simply if its support arm has its rear pivot located in the maximum ploughing width position, on the same side of the longitudinal plane of symmetry as the forward-most body, and is not far from said body, which, because of cantilevering, makes any beam weak that supports the bodies positioned thereon behind said pivot, and all the more so when the number of bodies is high.